register_function(self,
method,
external_name)

dispatch(self,
json_data,
**kwargs)

Verifies that the passed json encoded string
is in the correct form according to the json-rpc spec
and calls the appropriate method
Checks:
1. that the string encodes into a javascript Object (dictionary)
2. that 'method' and 'params' are present
3. 'method' must be a javascript String type
4. 'params' must be a javascript Array type
Returns:
the JSON encoded response